1. Field of the Invention
The present invention relates to recovery method of nickel, and particularly to recovery method of nickel which can recover and reproduce nickel in a metal form from solutions prior to treatment such as spent electroless plating solutions by electrolysis.
2. Background of the Invention
Recently due to rapid development of domestic industries, cases of coating a metal layer to various material surfaces have increased in order to provide new functions such as corrosive prevention and conductivity or provide beautiful metal gloss. Metal plating process is one of important methods for the above metal coating, which has been used for much long time already and separate from the electroplating, application of electroless metal plating has continued to increase also.
As the electroless nickel plating has characteristics that it is possible to plate a nickel layer very evenly to the surface of non-conductive materials such as plastics, glasses, and ceramic materials and improve physical properties of the materials such as abrasion resistance, it is often applied to treatment of various materials for electro vehicle parts and electro-devices.
However, in case of the electroless plating, composition and ingredients of the plating solution is featured by getting further complicated than general electroplating, accordingly the spent electroless plating solution may cause serious environmental pollution. Especially, electroless nickel plating solution includes considerable amount of various organic acids and organic salts as complexing agents to produce complex compounds with nickel ions as well as sodium hypophosphite (NaH2PO2) used as a reducing agent, so these spent solution is more likely to cause environmental pollution.
As a treatment method of the spent electroless nickel plating solution, a method to add lime to the waste solution to remove various ions in the waste solution by making them precipitated as calcium salts has been mainly used. But the above precipitation method using lime has problems that not only is difficult to remove nickel completely but also produces large amount of sludge. In order to these demerits, oxidation methods using various oxidizing agents had been developed, but it has been reported that in that case, there were some problems in economic efficiency because of high price of the oxidizing agents.
In addition, the above mentioned conventional treatment methods of spent electroless plating solution have fundamental problems that they were developed simply for treatment of waste solution, so treat nickel, the available metal resource, simply as a waste and cannot recover and reproduce it as a product.
Methods to recover and reproduce nickel as a product from the electroless plating solution include a method to add alkali such as caustic soda to the waste solution to recover nickel by making it precipitated as nickel hydroxide and a method to recover nickel as metal form by electrolysis of the waste solution. The former has demerits that because very strong alkali solution over pH 14 is required to make the nickel stabilized as a complex compound precipitated as nickel hydroxide, the precipitated nickel hydroxide has very low purity from adulteration of phosphorus and various organic matters as impurities as well as requiring excessive amount of caustic soda. The latter to recover nickel by direct m electrolysis of the spent electroless plating solution has also a demerit that because nickel ions in the waste solution are combined stably with the complexing agent, electrodeposition of nickel is highly difficult in general electrolysis conditions.